The present invention relates to a process for recovering boron trifluoride from an impure boron trifluoride residue, and more particularly, to a process for separating boron trifluoride from an impure boron trifluoride residue having sulfur dioxide therein.
As taught by commonly assigned U.S. Pat. No. 2,416,133, boron trifluoride (hereinafter BF.sub.3) is conveniently manufactured by the reaction of boric acid (hereinafter H.sub.3 BO.sub.3) with fluorosulfuric acid (hereinafter FSO.sub.3 H) by using sulfuric acid (hereinafter H.sub.2 SO.sub.4) as the reaction medium. EQU H.sub.3 BO.sub.3 +3FSO.sub.3 H.fwdarw.BF.sub.3 +3H.sub.2 SO.sub.4
Fluorosulfuric acid is also known as fluorosulfonic or fluosulfonic acid.
BF.sub.3 may also be prepared by the reaction of H.sub.3 BO.sub.3 with hydrogen fluoride (hereinafter HF) followed by dehydration of the subsequently formed BF.sub.3 .multidot.hydrate with oleum. EQU H.sub.3 BO.sub.3 +3HF.fwdarw.BF.sub.3 .multidot.H.sub.2 O+2H.sub.2 O EQU BF.sub.3 .multidot.H.sub.2 O+2H.sub.2 O+Oleum(H.sub.2 SO.sub.4 +SO.sub.3).fwdarw.BF.sub.3 +H.sub.2 SO.sub.4
The foregoing reactions are followed by a distillation to separate pure BF.sub.3 from the impure BF.sub.3 residue.
BF.sub.3 is a Lewis acid or electron acceptor and is used primarily as an acidic catalyst for such reactions as polymerization, esterification, and alkylation. More specifically, BF.sub.3 is a useful catalyst for the polymerization of olefins, vinyl ethers or esters, heterocyclic unsaturated organic compounds, unsaturated acids or esters, and terpenes or derivatives thereof.
Although the aforementioned pure BF.sub.3 is useful in the foregoing applications, the impure BF.sub.3 residue is typically unacceptable because the level of impurities present is high. Regardless of the preparation method of BF.sub.3, the raw materials, i.e. FSO.sub.3 H, H.sub.2 SO.sub.4, oleum, and HF, used in BF.sub.3 preparation contain impurities which become part of the crude BF.sub.3 product stream. The impurity level must be reduced prior to the sale of BF.sub.3 because impurities such as sulfur trioxide (hereinafter SO.sub.3) and sulfur dioxide (hereinafter SO.sub.2) may react detrimentally in customer processes. For example, SO.sub.2 and SO.sub.3 may cause undesirable colored products while SO.sub.2 may also create a poisoned catalyst. In addition to SO.sub.2 and SO.sub.3, other impurities which may be present in very low concentrations include arsenic fluoride (hereinafter AsF.sub.5), antimony fluoride (hereinafter SbF.sub.3), and silicon fluoride (hereinafter SiF.sub.4). An analysis of a typical impure BF.sub.3 waste side steam or residue by weight is as follows: about 40 to about 95% BF.sub.3, about 5 to about 30% SO.sub.2, about 0 to about 19% SO.sub.3, about 0 to about 0.2% SiF.sub.4, about 0 to about 1.0% AsF.sub.5, and about 0 to about 0.1% SbF.sub.3.
In the industry, the sales specifications for typical quality BF.sub.3 require SO.sub.3 levels of about 0.05 to 0.001%. The SO.sub.3 typically reacts with moisture present in the system, and therefore, is seldom a problem. In the industry, the limits for SiF.sub.4 are about 0.03%. No industry specifications exist for the other impurities, and indeed, they are generally below accepted detection levels. SO.sub.2 is the most difficult impurity to remove from an impure BF.sub.3 residue. In the industry, the sales specifications for typical quality BF.sub.3 require SO.sub.2 levels of about 0.1 to 0.002%.
As such, purification of such an impure BF.sub.3 waste side stream or residue is necessary if the BF.sub.3 is to be isolated as a useful product. Since about 1952, the commercial practice has been to convert the impure BF.sub.3 residue by reaction with diethyl ether to form an impure BF.sub.3 -diethyl ether complex. This complex is then purified and used in the production of leaded gasoline and as a liquid source of BF.sub.3 for catalyzing various other reactions. Because the market for leaded gasoline has dropped sharply, this method of using the impure BF.sub.3 residue is no longer as commercially appealing. If the impure BF.sub.3 residue is sent to waste disposal rather than purified, the process yields are reduced and the process costs are increased due to the cost of waste treatment.
BF.sub.3 and SO.sub.2 are very soluble in water but BF.sub.3 .multidot.hydrate is difficult to decompose without destroying the BF.sub.3. BF.sub.3 and SO.sub.2 are also known as mentioned earlier, is a solvent used in the manufacture of BF.sub.3. It is also known that BF.sub.3 may be absorbed by many complexing agents which do not absorb SO.sub.2 but it is difficult to decompose such a complex in order to recover the BF.sub.3.
Commonly assigned U.S. Pat. No. 4,265,871 deals with the purification of the waste H.sub.2 SO.sub.4 which results from a BF.sub.3 preparation process. In the first step of the process, the contaminated H.sub.2 SO.sub.4 is contacted with an inert gas to remove BF.sub.3 from the H.sub.2 SO.sub.4. In the second step, the BF.sub.3 rich gas is passed through an absorbing mixture of boric and sulfuric acids so that the BF.sub.3 is recovered from the inert gas; the reference does not teach that SO.sub.2 is present in the waste H.sub.2 SO.sub.4 treated or address the problem which is solved by the present process.
Any method for recovering BF.sub.3 from an impure gas stream must permit recovery for either storage as product or else for recycling to process for subsequent recovery as a saleable product. It would be convenient to be able to purify an impure BF.sub.3 stream by destroying SO.sub.2 so as to free BF.sub.3 from its major contaminant; alternatively, it would be convenient to absorb SO.sub.2 or BF.sub.3 so as to leave a relatively pure stream of BF.sub.3 for recovery or a stream of SO.sub.2 for sending to waste treatment.
As such, the industry needs a simple means for processing an impure boron trifluoride residue wherein a commercially useful product is generated and waste disposal problems of current processes are avoided.